This project investigates the neuroendocrine mechanisms by which mammals generate seasonal variations in physiological and behavioral traits in synchrony with changing environmental conditions and the consequences of this variation for aging of circadian function. Studies in this area have examined responses of Siberian, Phodopus sungorus, and Syrian hamsters, Mesocricetus auratus, exposed to static long and short day lengths of early summer and winter, respectively. Accumulating evidence establishes that photoperiodic systems function markedly differently under natural conditions that incorporate the incrementally and continuously changing pattern of day lengths under which these systems evolved. An ecologically relevant understanding of these annual rhythms is particularly important clinically, as humans, like laboratory rodents, are commonly isolated from the yearly pattern of change in daylength, but the consequences of this deprivation have gone virtually unexamined. Specific experiments in hamsters address 1) whether under naturalistic conditions melatonin interacts endogenous temporal programs that mediate the transitions between photoperiodic states;2) how the function of the circadian oscillator is regulated by seasonal photoperiods;and 3) how age-related deficits in circadian function can be retarded by photoperiodic manipulations. The proposed experiments are relevant to an understanding of the myriad human seasonal rhythms that have been previously documented (e.g., in cardiovascular disease, neurodevelopmental disorders, immune function, sleep duration, body weight, depression and nonpsychiatric mood states). Additionally, these studies will assess whether photoperiodic manipulations might be effective adjuncts to achieve rapid phase-shifting of human rhythms, and whether such manipulations have therapeutic value for disruptions of circadian rhythmicity in old age. Finally, these studies form a necessary information base to evaluate the potential physiological costs and benefits of artificial lighting regimes experienced by humans in modern society.